Using the large Kaiser-Permanente Twin Registry, we are carrying out a three-phase study of atherosclerotic disease and risk factors in adult like-sex monozygotic and dizygotic twins. The main objectives are to assess the degree of genetic determination of atherosclerotic disease, particularly coronary heart disease (CHD) and its risk factors, and to assess the role of environmental factors in CHD, with emphasis on cigarette smoking and alcohol consumption. The first phase is an analysis of responses to a questionnaire on health status, habits, and environmental exposures. The second is a review and analysis of medical records of those twins who are Kaiser Foundation Health Plan subscribers. The third phase will be a medical examination of 432 pairs of female twins including extensive laboratory tests. Long-term follow-up of participants is also planned.